feat(operations): backup-to-template grouping fix + artifact/backup delete

_template_of_backup only stripped one "_"-chunk off the archive name,
so real Template_<name>_%Y%m%d_%H%M%S.7z files (date *and* time) never
matched their template -- backups silently failed to show up grouped
under it. Replaced with a regex that strips the full optional
date+time suffix and tolerates underscores in template names.

Also adds delete_artifact/delete_backup (mirrored, containment-checked
single-file deletion) and backups_by_template (groups list_backups()
output by parsed template name, newest-first) for the web dashboard's
per-item delete actions and the Templates panel's per-row backup list.

tests/python/test_operations_inventory.py

@@ -162,6 +162,27 @@ def test_list_templates_last_backup_picks_newest_matching_archive(tmp_path: Path
     assert info.last_backup.name == "Template_LinuxBuild2404_20260601_000000.7z"
 
 
+@pytest.mark.parametrize(
+    ("filename", "expected"),
+    [
+        # Real archive shape: ``Template_<name>_%Y%m%d_%H%M%S.7z`` -- both the
+        # date and the time chunk must be stripped (regression: only the time
+        # chunk was, leaving "LinuxBuild2404_20260722").
+        ("Template_LinuxBuild2404_20260722_145001.7z", "LinuxBuild2404"),
+        ("Template_WinBuild2025_20260101_000000.7z", "WinBuild2025"),
+        # Template names containing underscores survive intact.
+        ("Template_Win_Build_2025_20260722_145001.7z", "Win_Build_2025"),
+        # Date-only shape (no time chunk) is still tolerated.
+        ("Template_LinuxBuild2404_20260722.7z", "LinuxBuild2404"),
+        # Non-matching names parse to None.
+        ("not-a-template-archive.7z", None),
+        ("Template_no_timestamp.txt", None),
+    ],
+)
+def test_template_of_backup_strips_full_timestamp(filename: str, expected: str | None) -> None:
+    assert inventory_module._template_of_backup(filename) == expected
+
+
 def test_list_templates_last_backup_none_when_no_backups_dir(tmp_path: Path) -> None:
     (tmp_path / "templates" / "LinuxBuild2404").mkdir(parents=True)
     config = _make_config(tmp_path)